How important a role did William Seward play in winning the Civil War and saving the Union? According to Lincoln’s assassin, John Wilkes Booth, an extremely important one.

A Shakespearean actor, Booth knew the play Julius Caesar well and appreciated that Brutus had failed to restore the Roman republic because he killed only Caesar and let Marc Antony live. To Booth, both men were tyrants, and he knew “other brains” than the president ran the country. Killing Lincoln would not suffice; Seward had to die, too.

The gripping account of the failed assassination attempt on Seward at his Lafayette Square house begins Walter Stahr’s new book, Seward: Lincoln’s Indispensable Man. The book provides the first full-volume biography of the secretary of state in years. It makes the case that Booth’s assessment of Seward’s importance in the Lincoln administration was not misplaced.

Lincoln’s one-time presidential rival proved to be a steady hand and a shrewd mind at the State Department. Lincoln increasingly learned to rely on Seward’s advice. Perhaps the best example of Seward’s nimble navigation through troubled geopolitical waters came early in the war when the U.S. Navy arrested two Confederates who had been on board a British ship. The Trent Crisis, as it became known, evolved into the “Cuban missile crisis of the 19th century: a moment when the United States faced possible war with the world’s other major power, a period of intense emotion and apprehension.”

After news broke that two Confederates had been captured at sea, euphoria swept across the North, which had been looking for some good news to cheer. But the British Empire was none too happy that its ship had been commandeered by American sailors. In the 19th century, this could be construed as an act of war.

Stahr performs two great tasks in his retelling of the Trent Crisis. First, he provides a play-by-play account and shows Seward carefully choreographing events. Lincoln found himself trapped between the euphoria of the North and the rage of the British Empire. Almost from the start, Seward sensed that the only way out would be a diplomatic about-face to release the Confederate prisoners. He began quietly discussing the matter with the president until Lincoln could resist no more.

“I found I could not make an argument … and that proved to me your ground was the right one,” Lincoln told Seward when he agreed to release the Confederates. War with Britain was averted.

Stahr also points to the historical record to show that Seward’s role in this crisis has not always been correctly told. For example, Seward has often been accused of being excited about the initial capture of the Confederates. Stahr traces this belief back to Gideon Welles, the secretary of the Navy and a rival to Seward. Stahr claims that there is “no contemporary evidence … that Seward was elated; on the contrary, he was almost instantly concerned.”

Writing like that makes history come alive: a researcher digging into the mines of the past and quarrying new insight on an old story. Seward: Lincoln’s Indispensable Man is filled with these stories powerfully told by a historian who has provided a great book worthy of a great man.

Kasey S. Pipes serves as the Norris senior fellow at the Eisenhower Institute of Gettysburg College and wrote Ike’s Final Battle: The Challenge of Equality and the Road to Little Rock.